Problem: What is the greatest common factor of $9$ and $18$ ? Another way to say this is: $\operatorname{gcf}(9, 18) = {?}$
Solution: The greatest common factor is the largest number that is a factor of both $9$ and $18$ The factors of $9$ are $1$ $3$ , and $9$ The factors of $18$ are $1$ $2$ $3$ $6$ $9$ , and $18$ Thus, the greatest common factor of $9$ and $18$ is $9$. $\operatorname{gcf}(9, 18) = 9$